This simple but useful module allows users to "join" a role just using a given password.
Site administrator can decide WHAT ROLE users should be able to join using a password, and WHAT USERS should be able to use this feature. Users can join selected roles, if they know the correct password; they can then leave that role, also if they dont know the password.
I use it in conjunction with the Forum Access module, to make easier for [many] users to "join" a private forum: instead of manually give them the proper role, I give them the password and they join (and leave) the role by theirself.
You could also check the Apply_for_role module: it does almost the same thing, but it requires that administrator manually approve each application (not easy to do when you have many users on your website).
Follow updates also on my twitter channel
(please use the "dev" version until next release update)